1. Field of the Invention
The present invention relates generally to the field of processing of borehole seismic data. More specifically, the present invention discloses a method for processing borehole seismic data into the form of surface seismic data so that conventional surface seismic data processing methods can then be applied to form a subsurface image of the earth.
2. Statement of the Problem
The widely known and widely used art of surface seismology involves placing seismic sources and seismic receivers at the surface of the earth and recording seismic waves that originate at the seismic source point. As illustrated in FIG. 1, a conventional method of collecting seismic data in surface seismic operations is to place seismic sources and seismic receivers at the surface of the earth. Each seismic source is initiated and the seismic wavefield from the individual seismic sources is recorded on multiple receivers. Data recorded by geophones (also known as seismometers) at the surface of the earth can be processed by widely known methods (see, Yilmaz, O., Seismic Data Processing, (Society of Exploration Geophysicists, 1987)) to obtain an image of the interior of the earth.
One of the most commonly used processing methods is called CDP (Common Depth Point) processing. In this method, sources and receivers from different common source gathers (see FIG. 1) are sorted into common midpoint gathers, otherwise known as CDP (Common Depth Point) gathers, as illustrated in FIG. 2.
Reflections in a CDP gather are hyperbolic in the time-offset plane, as shown in FIG. 3, where the word “offset” is used to describe the horizontal distance from the source to the receivers. The trace on the left side of the gather in FIG. 3 has an offset of zero, in other words the source and receiver were coincident in space at the time of recording. The time delay of reflections with increasing offset is due to the increased seismic wave travel path with increased source-receiver separation in the horizontal direction.
A mathematical operation known as Normal Moveout (NMO) can be applied to the reflections in a CDP gather to correct reflection travel times so that the reflection time after application of NMO is equivalent to the travel time at zero-offset, i.e., where the source and receiver were coincident at the surface of the earth at the time of recording. FIG. 4 shows a synthetic common depth point gather in FIG. 3 after correction for NMO.
Having both sources and receivers at the surface of the earth is required for two key aspects of this reflection seismology technique to work properly. The two aspects are: (1) to first order, the spatial point from which a seismic reflection originates can be assumed to be half way between the source and receiver; and (2) the shape of a reflection in the time-offset plane is hyperbolic and can be predicted by the NMO equation. The assumptions of these two key aspects are violated in proportion to the degree that reflecting interfaces in the subsurface dip (or tilt) from flat lying. But even with steep dips, the earth can be imaged with well-developed surface seismic techniques.
A sub-field of reflection seismology is borehole seismology in which seismic receivers are placed in one or more boreholes in the subsurface and source points are at the surface of the earth, as shown in FIG. 5. This type of data is generally known as Offset VSP (Vertical Seismic Profile) data, but is also alternatively known as 2D VSP or 3D VSP data. Alternatively the source can be in the borehole with receivers at the surface of the earth. The borehole seismic source can be of any type, including data derived from using a drilling bit as the seismic source. This technique is commonly known as Reverse VSP.
There are significant advantages to recording seismic data by VSP methods, not the least of which is increased seismic frequency content over that which can be recorded at the surface of the earth. Therefore, the potential exists to obtain greater geologic detail from the data. The significant disadvantage however is that the symmetry of having source and receivers at the same elevation is lost. Thus, the common midpoint reflection point assumption is lost and the NMO equation does not apply. Further, there is not currently an analogous equation for midpoint determination and moveout correction to apply to offset VSP data.